The present invention relates to an additive and a process for inhibiting nucleation, growth and/or agglomeration of gas hydrates by adding an effective amount of an inhibitor which contains esters of alkoxylated hydroxycarboxylic acid amides to a multiphase mixture tending to hydrate formation and comprising water, gas and possibly condensate or to a drilling fluid tending to gas hydrate formation.
Gas hydrates are crystalline clathrate compounds of gas molecules in water, which form under specific temperature and pressure conditions (low temperature and high pressure). The water molecules form cage structures around the corresponding gas molecules. The framework formed from the water molecules is thermodynamically unstable and is stabilized only by the inclusion of gas molecules. Depending on pressure and gas composition, these ice-like compounds can also exist above the freezing point of water (up to more than 25° C.).
In the petroleum and natural gas industry, in particular the gas hydrates which form from water and the natural gas components methane, ethane, propane, isobutane, n-butane, nitrogen, carbon dioxide and hydrogen sulfide are of considerable importance. Particularly in present day natural gas production, the existence of these gas hydrates presents a major problem especially when wet gas or multiphase mixtures comprising water, gas and alkane mixtures under high pressure are exposed to low temperatures. Here, owing to their insolubility and crystalline structure, the formation of gas hydrates leads to blockage of a very wide range of transport means, such as pipelines, valves or production systems in which wet gas or multiphase mixtures are transported over long distances at low temperatures, as occurs in particular in colder regions of the world or on the seabed.
In addition, gas hydrate formation can lead to problems also in the drilling process for developing new gas or petroleum deposits under corresponding and temperature conditions by virtue of the fact that gas hydrates form in the drilling fluids.
To avoid such problems, the gas hydrate formation in gas pipelines, during transport of multiphase mixtures or in drilling fluids can be suppressed by using relatively large amounts (more than 10% by weight, based on the weight of the aqueous phase) of lower alcohols, such as methanol, glycol or diethylene glycol. The addition of these additives of these additives shifts the thermodynamic limit of gas hydrate formation to lower temperatures and higher pressures (thermodynamic inhibition). However, the addition of these thermodynamic inhibitors gives rise to greater safety problems (flash point and toxicity of the alcohols), logistic problems (greater storage tanks, recycling of these solvents) and accordingly high costs, especially in offshore production.
Today, attempts are therefore being made to replace thermodynamic inhibitors by adding, at temperature and pressure ranges in which gas hydrates can form, additives in amounts of <2% which either delay the gas hydrate formation (kinetic inhibitors) or keep the gas hydrate agglomerates small and therefore pumpable so that they can be transported through the pipeline (so-called agglomerate inhibitors or anti-agglomerates). The inhibitors used either prevent the nucleation and/or the growth of the gas hydrate particles or modify the hydrate growth in such a way that smaller hydrate particles result.
In addition to the known thermodynamic inhibitors, a large number of monomeric as well as polymeric classes of substances which are kinetic inhibitors or agglomerate inhibitors have been described as gas hydrate inhibitors in the patent literature. Of particular importance here are polymers having a carbon backbone which contain both cyclic (pyrrolidone or caprolactam radicals) and acyclic amide structures in the side groups. Surface-active compounds having both nonionic and cationic structures are also known for use as a gas hydrate inhibitor.
To be able to use gas hydrate inhibitors also with greater supercooling than currently possible, i.e. further within the hydrate region, further increase of the effectiveness compared to the hydrate inhibitors of the prior art is required.
DE-C-199 20 152 describes modified polyglycol-ethercarboxylic acid-amides as additives for inhibiting gas hydrate formation, which, depending on the structure, exhibit broad activity and prevent both the nucleation and the growth or the agglomeration of gas hydrates. However, the compounds disclosed have poor biodegradability, which prevents their use especially in offshore applications.
It was therefore an object of the present invention to provide improved additives which are completely biodegradable and have a broad application spectrum with high activity potential, both for slowing down the formation of gas hydrates (kinetic inhibitors) and for keeping the gas hydrate agglomerates small and pumpable (anti-agglomerates).